A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model, and the term layer 2 switch is often used interchangeably with bridge. Bridges are similar to repeaters or network hubs, devices that connect network segments at the physical layer; however, a bridge works by using bridging
where traffic from one network is managed rather than simply rebroadcast
to adjacent network segments. In Ethernet networks, the term "bridge"
formally means a device that behaves according to the IEEE 802.1D standard—this is most often referred to as a network switch in marketing literature.
Since bridging takes place at the data link layer of the OSI model, a bridge processes the information from each frame of data it receives. In an Ethernet frame, this provides the MAC address of the frame's source and destination. Bridges use two methods to resolve the network segment that a MAC address belongs to.
- Transparent bridging
– This method uses a forwarding database to send frames across network
segments. The forwarding database is initially empty and entries in the
database are built as the bridge receives frames. If an address entry
is not found in the forwarding database, the frame is rebroadcast to
all ports of the bridge, forwarding the frame to all segments except
the source address. By means of these broadcast frames, the destination
network will respond and a route will be created. Along with recording
the network segment to which a particular frame is to be sent, bridges
may also record a bandwidth metric to avoid looping when multiple paths
are available. Devices that have this transparent bridging
functionality are also known as adaptive bridges. They are primarily found in Ethernet networks.
- Source route bridging
– With source route bridging two frame types are used in order to find
the route to the destination network segment. Single-Route (SR) frames
comprise most of the network traffic and have set destinations, while
All-Route(AR) frames are used to find routes. Bridges send AR frames by
broadcasting on all network branches; each step of the followed route
is registered by the bridge performing it. Each frame has a maximum hop
count, which is determined to be greater than the diameter
of the network graph, and is decremented by each bridge. Frames are
dropped when this hop count reaches zero, to avoid indefinite looping
of AR frames. The first AR frame which reaches its destination is
considered to have followed the best route, and the route can be used
for subsequent SR frames; the other AR frames are discarded. This
method of locating a destination network can allow for indirect load balancing
among multiple bridges connecting two networks. The more a bridge is
loaded, the less likely it is to take part in the route finding process
for a new destination as it will be slow to forward packets. A new AR
packet will find a different route over a less busy path if one exists.
This method is very different from transparent bridge usage, where
redundant bridges will be inactivated; however, more overhead is
introduced to find routes, and space is wasted to store them in frames.
A switch with a faster backplane can be just as good for performance,
if not for fault tolerance. They are primarily found in Token Ring
networks.
Advantages of network bridges
- Self configuring
- Primitive bridges are often inexpensive
- Reduce size of collision domain by microsegmentation in non switched networks
- Transparent to protocols above the MAC layer
- Allows the introduction of management - performance information and access control
- LANs interconnected are separate and physical constraints such as number of stations, repeaters and segment length don't apply
Disadvantages of network bridges
- Does not limit the scope of broadcasts
- Does not scale to extremely large networks
- Buffering introduces store and forward delays - on average traffic
destined for bridge will be related to the number of stations on the
rest of the LAN
- Bridging of different MAC protocols introduces errors
- Because bridges do more than repeaters by viewing MAC addresses, the extra processing makes them slower than repeaters
- Bridges are more expensive than repeaters
Bridging versus routing
Bridging and
Routing are both ways of performing data control, but work through different methods. Bridging takes place at
OSI Model Layer 2 (Data-Link Layer) while Routing takes place at the
OSI Model Layer 3 (Network Layer). This difference means that a bridge directs frames according to hardware assigned
MAC addresses while a router makes its decisions according to arbitrarily assigned
IP Addresses. As a result of this, bridges are not concerned with and are unable to distinguish
networks while
routers can.
When
designing a network, you can choose to put multiple segments into one
bridged network or to divide it into different networks interconnected
by routers. If a host is physically moved from one network area to
another in a routed network, it has to get a new IP address; if this
system is moved within a bridged network, it doesn't have to
reconfigure anything.
Specific uses of the term "bridge"
Documentation on Linux bridging can be found in the Linux networking wiki Linux bridging allows filtering and routing.
Certain versions of Windows (including XP and Vista) allow for creating a Network Bridge - a network component that aggregates two or more Network Connections and establishes a bridging environment between them. Windows does not support creating more than one network bridge per system.
Filtering Database
To translate between two segments types, a bridge reads a
frame's destination
MAC address and decides to either forward or filter. If the bridge determines that the destination
node is on another segment on the network, it forwards it (retransmits) the
packet
to that segment. If the destination address belongs to the same segment
as the source address, the bridge filters (discards) the frame. As
nodes transmit data through the bridge, the bridge establishes a
filtering database (also known as a forwarding table) of known MAC
addresses and their locations on the network. The bridge uses its
filtering database to determine whether a packet should be forwarded or
filtered.
See also
External links